1. Field of the Invention
This invention relates to a process for preparing spinel extrudates, especially extrudates in honeycomb form, which, when dried and calcined, are useful as catalysts.
2. Description of the Prior art
The combustion of hydrocarbons with air as oxidant gives rise, particularly with excess air and high temperatures, to nitrogen oxides through oxidation of the nitrogen present in the air. Examples of such nitrogen oxides are NO, NO.sub.2, NO.sub.3, N.sub.2 O.sub.3, N.sub.2 O.sub.4 and N.sub.2 O.sub.5. Being pollutants, the nitrogen oxides should be removed as completely as possible from the combustion exit gases to avoid burdening the environment. Whereas power plant and industrial emissions are being progressively curtailed through the use of exit gas treatment facilities, abating the pollutant fraction in motor vehicle exit or exhaust gases is becoming more and more important, especially against the background of the increasing number of motor vehicles.
Many solutions have been proposed for abating NO.sub.x emissions from motor vehicle engines. Effective solutions for abating NO.sub.x levels have to meet a multiplicity of criteria, especially if catalysts are used, for example:
high conversion ratio, i.e., substantial NO.sub.x removal, even at high and low temperatures and in the event of frequent load changes during operation PA1 avoidance of the use of auxiliary materials such as ammonia or urea PA1 low manufacturing and operating costs PA1 long onstream time PA1 low N.sub.2 O production PA1 high mechanical catalyst stability. PA1 the coprecipitation from hydroxides with subsequent calcination, PA1 the mixing of the oxides and subsequent calcination, PA1 the use of old recycled honeycombs, PA1 the use of production scrap and residues, PA1 the mixing of the salts, spray drying and subsequent calcination.
A number of catalysts have been proposed for reducing nitrogen oxides. One train in the development of suitable catalysts is moving in the direction of spinel catalysts.
The use of copper-impregnated CuAl.sub.2 O.sub.4 spinels as exit gas catalyst is described in DE-A-195 46 482. The spinels are used in the form of spall.
EP-A-0 779 093 describes corresponding spinel catalysts for reducing nitrogen oxides and for oxidizing hydrocarbons. The spinels are based on zinc, copper and aluminum and are used in the form of spall.
EP-A-0 676 232 describes spinel catalysts useful for exit gas treatment, which abate nitrogen oxide levels in the exit gas. They are zinc aluminum spinels which are obtainable by precipitation from precursor solutions. The precipitation products can be dried by spray drying or flash evaporation of the solvent and are obtained as powders. It is also possible to mix the catalyst precursors with, for example, urea or glycine and to burn the mixture, in which case not only spinel formation but at the same time also, owing to the high temperature, a calcination takes place. It is stated that the catalysts can be present in the form of honeycombs, but a manufacturing process for honeycomb structures is not described.
Spinel catalysts are preferably used in motor vehicles not in the form of spall or granulate, but in the form of honeycomb structures which consist of the catalyst material and possess a number of essentially parallel channels through which the gas to be treated flows. In cross-sectional view, such honeycomb structures may correspond to honeybee combs, for example. The individual channels may also have a round or rectangular or especially square cross section, so that the cross section through the honeycomb structure corresponds to a right-angled grid pattern. Various processes have been proposed for preparing spinel honeycomb structures.
DE-C-36 19 337 describes a process for preparing TiO.sub.2 --SiO.sub.2 oxide compounds which may further contain zirconium dioxide. An aqueous solution comprising active constituents such as vanadium and copper, or a powder of the active constituents in the form of the oxides is added together with a molding assistant to a titanium-containing oxide, such as TiO.sub.2, TiO.sub.2 --SiO.sub.2 and TiO.sub.2 --SiO.sub.2 --ZrO.sub.2. The constituents are then mixed and kneaded while a suitable amount of water is added. The mixture is then molded in an extruder molding apparatus. The molded product is dried and calcined.
DE-A-44 19 974 relates to a process for preparing a molded catalyst based on titanium oxide and its use. The catalyst can be in the form of a honeycomb structure. It is prepared by kneading calcined titanium dioxide powder in a mixture with water, an ammonium metatungstate solution, polyethylene glycol as plasticizing assistant, monoethanolamine and glass fibers. This homogeneous kneaded material is molded in an extrude into honeycombs and the honeycomb catalysts are subsequently dried and calcined.
U.S. Pat. No. 5,219,816 relates to dehydrogenation catalysts and process for their preparation. For preparing the support which is based on a spinell, magnesium nitrate and an aluminium oxide material are mixed and extruded into a suitable mold. After drying calcination is performed at temperatures from 600 to 700.degree. C.
The preparation of honeycomb structures, especially copper-aluminum spinels, by existing processes has a number of disadvantages.
The extrusion of the molding material into honeycomb structures requires a long feed time, which corresponds to a low feed speed. Frequently, only feed speeds of 5 to 10 cm/min are achieved.
The drying time for the extruded honeycomb structures is very long. It is frequently at least one month at room temperature.
As they dry and in the course of the subsequent calcination, the honeycomb structures are generally prone to undergo distortion, for example parallelogram distortion and pincushion distortion, and develop longitudinal and transverse cracks. Even prolonging the drying time does not led to better results.
Such honeycomb structures are mechanically unstable because of the cracks and distortions, and lead to a nonuniform gas flow through the catalyst, so that it is difficult or even impossible for constant reaction conditions to be established over the catalyst. In addition, for example in motor vehicle catalysts, the honeycomb structures are fitted into an overcoat. When parallogram distortions and pincushion distortions are present, the honeycomb structures will no longer finish flush with the overcoat, so that exhaust gas can bypass the catalyst, greatly reducing catalyst performance as a whole. In this case, the catalyst considered as a whole thus exhibits diminished activity.